This invention relates to cutters for forming, at an open end of a bore or the like, a countersink or spot facing or other such ring-form surface of revolution concentric to the bore. It is particularly concerned with cutter attachments that can be adjustably assembled with a drill bit or other generally cylindrical leading element producing or fitting the bore.
The use of such cutter attachments mounted upon tools such as twist drills is well known as a means of drilling and countersinking a hole in a single operation. As examples can be mentioned British Patents 1378869, 1378868, 1304382, 988263, 570099 and 570098.
These earlier cutter attachments have disadvantages, however, particularly if they are to be used at the high rates of material removal associated with current cutting technology. They have a tubular body with a central bore in which the associated drill bit can slide for the purpose of axial adjustability, the countersink cutting edges on the body projecting outwardly at the sides of the bore. These cutting edges have a limited operating life before resharpening is required, since they are formed from the same material as the cutter body. Some improvement in this respect is obtained by providing cutter bodies with brazed-on tungsten carbide tips but with all forms of this type of cutter attachment there are high servicing costs because regrinding and resetting is both expensive and time-consuming.
In U.S. Pat. No. 4353670 this problem is addressed by providing a special form of drill bit or other carrier for the cutter attachment, a flat recess being formed on the otherwise generally cylindrical carrier and the attachment is arranged to be clamped on the flat recess. A cutting insert of the attachment has its main body disposed partially in the recess or, in the case of a twist drill, in a drill flute that forms an extension of the recess. It is a disadvantage, however, only to be able to use the attachment on a specially adapted carrier.